Development Path of CFP2
Since 2010, we have been early 100G client deployment base on the CFP form factor. The module host interface was based on a more conservative 10x10G interface base on established 10G technology. While the CFP form factor is flexible( supporting a wide range of technologies, from 40G serial through 40G parallel and 100G in both 4x25G and 10x10G,) its large form factor and relative complex technology keeps it from meeting and density and price expectation required for mainstream 100G.
Figure 1 shows the 100G client-side form-factor evolution. The CFP2 form factor realizes the density at a price for more mainsream applications. It shares the same photonic interface as the 100G CFP at twice the faceplate density. The host interface moves from 10x10G to 4x25G. It also continues to use the MDIO control interface that the CFP uses .
When the first 100G CFPs appeared in 2008, 10G data rate electronics were relatively well established, making the host interface(10x10G) relatively risk-free compared to the 25G required for the photonics. The CFP required a so-called “gearbox”IC to multiplex/demultiplex the 10x10(CAUI) host interface into the 4x25G lanes that the photonics required. The 25G data path was limited to the short distances within the CFP module elimination the need to traverse pluggable connector or longer traces on the host /module boundary.
Using 10G technology allowed for connector and PCB designs to be based on existing practices. Challenges on issues such as dynamic skew and signal integrity have been overcome allowing early adopters to use CFP. The transition to CFP2 requires 25G data-rate signals to traverse the pluggable host/module interface, likely including several inches of PCB trace, presenting a considerable challenges as most 25G ICs are still first generation; and we lack the extensive knowledgeand experience we have with 10G. We expect to face the challenges we experienced with the CFP at 10G(crosstalk, signal integrity, and dynamic skew), but they will be compounded with challenges of the 25G data rate.